CA1251696A

CA1251696A - Polyoxometalate-modified carbon electrodes and uses therefor

Info

Publication number: CA1251696A
Application number: CA000521299A
Authority: CA
Inventors: Edward A. Cuellar; Michael A. Desmond
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1985-11-22
Filing date: 1986-10-24
Publication date: 1989-03-28
Also published as: EP0228168A1; KR900005444B1; JPS62194609A; KR870005424A; US4630176A

Abstract

ABSTRACT
IMPROVED POLYOXOMETALATE-MODIFIED CARBON ELECTRODES AND USES THEREFOR
Improved activated carbon electrodes are disclosed which electrodes have increased charge storage capacity and reduced leakage current. Such improved electrodes are incorporated into energy storage devices such as electric double layer capacitors. The improved charge storage capacity is due to the adsorption of polyoxometalate compounds onto the activated carbon and reduced leakage current is achieved by stabilizing the polyoxometalate compounds within the activated carbon through the use of a compound capable of generating at least one ionic site.

Description

lZS~
1 (85-P-0966) IMPROVED POLYOXOMETALATE-MODIfIED CARBON EUECTRODES AND USES THEREfOR

Field of the Invention The present lnvention relates to the modiflcation of actlvated carbon-based electrodes with polyoxometalate compounds. The modlfled electrodes possess greater charge storage denslty than unmodifled carbon electrodes as the polyoxometalate does not detract from the charge storage functlon of the carbon electrode but does store charge through a secondary reactlon mechanlsm.

8ackqround of the Invention Energy generatlon and storage has long been a sub~ect of study and development Of speclal importance is the storage of energy in a compact portable system that can be easlly charged and dlscharged such as rechargeable batterles and capacltors. Indlvidual components of such systems have been indlvidually lnvestlgated and optlmized generally by seeking to achleve a maxlmum stored energy density. However most commerclally explolted systems yield far less than their theoretical energy density.
One such energy storage system utillzes activated carbon electrodes to store ions therein which upon discharge release the ions to generate an electrlcal current. An example of an actlvated carbon electrode system is the electric double layer capacitor system descrlbed in U.S. 3 536 963 entitled Electrolytic Capacltor Having Carbon Paste Electrodes to Boos. The mechanism for energy storage is based on the "" - 2 1ZS169~ ~8S-P-0966) formatlon of an electrlcal double layer at the lnterface between an actlvated carbon electrode and a supportlng electrolyte under an applled electrlc fleld. These devlces wlll accept and store signiflcant amounts of energy at any avallable potentlal over whlch the devlce ls stable unllke batterles where a given threshold voltage must be exceeded.
Optlmlzation of thls system ls based on the optlmlzatlon of the charge storage capaclty of the actlvated carbon electrode used thereln. It has been found that the capacity of such an electrlc double-layer capacltor can amount to several tens of farads per gram of actlvated carbon when the actlvated carbon has a surface area in excess of 1000 m /9.
U.S. Patent No. 4,633,372 entitled Polyoxometalate - Modlfled Carbon Electrodes and Uses Therefor descrlbes lmproved actlvated carbon electrodes havlng enhanced charge storage density. As descrlbed thereln a polyoxometalate compound ls adsorbed into actlvated carbon to signiflcantly lmprove the charge storage capaclty of a device whlch lncorporates the modlfied carbon as an electrode thereln. Polyoxometalate specles capable of undergolng reverslble multlple electron reductlon-oxldation steps over the range of potentials applied to the energy storage devlce exhibit an electrochemical response resembllng the charge characteristics of the activated carbon electrode. Thls comblnation leads to devices having enhanced charge storage capabllltles wlth discharge curve characteristics slmilar to a like device using an unmodified activated carbon electrode.
An electrlc double layer capacltor as dlsclosed ln U.S. patent no. 3 536 963 ~hich lncorporates polyoxometalate-modlfied carbon 3 (85-P-0966) electrodes exhlbits charge storage capacltles that are up to about one-hundred percent greater than the charge storage capacity of a llke devlce which utllizes unmodlfied carbon electrodes.
However lt has been found that electrlc double layer capacltors utllizlng a mlcroporous separator and incorporatlng these polyoxometalate-modlfled carbon electrodes also exhibit undesirably hl~h leakage currents. Leakage current as used hereln ls the contlnuous current that passes through a fully charged cell to maintaln steady state voltage condltlons. Leakage current can be measured by the amount of current requlred to maintaln a constant voltage in the cell after full charge is obtained. The leakage current of electric double layer capacltors utillzing polyoxometalate-modlfled carbon electrodes is hlgh for very low current appllcations such as computer back-up appllcations. This lncreased leakage current may be due to transfer of polyoxometalate through the cell s microporous separator. The use of a cation-exchange membrane about eliminates the increased leakage current but such membranes are expensive and more difficult to process than a mlcroporous separator materlal.
Thus it would be a technical improvement to provlde a polyoxometalate-modified activated carbon wherein the polyoxometalate was stablized on the activated carbon.
It ls therefore one ob~ect of the present invention to provlde a polyoxometalate-modified activated carbon electrode having a relatlvely hlgh charge storage capaclty wherein the polyoxometalate is stably disposed on the activated carbon.

. 4 ~ ~ S ~ ~85-P-0966) It ls another object of the present lnvention to provide a process for stably disposlng a polyoxometalate compound on an actlvated carbon.
It ls yet a further ob~ect of the present invention to provide an electric double layer capacitor lncorporating stably-disposed polyoxometalate-modlfled activated carbon electrodes thereln.
These and other ob~ects of the present lnventlon will become apparent to those skilled in the art from the below descrlption of the inventlon.

Summary of Invention The present invention relates to an lmproved carbon-based electrode whlch electrode comprlses an activated carbon electrode having a polyoxometalate compound stabllzed on the activated carbon surface by a compound capable of generatlng at least one lonic site. Preferably the compound ls capable of generating multiple ionlc sites. Most preferably the compound capable of generatlng at least one ionic slte ls an amlne.
The lnventlon also relates to a process for making an improved carbon-based electrode comprising the steps of contacting a polyoxometalate compound and a compound capable of generating at least one ionic slte ln the presence of activated carbon so as to stably dispose the polyoxometalate on the activated carbon. The process may occur by an lon exchange reaction or by an acid/base reactlon.
In addltlon the present invention relates to an energy storage devlce comprislng a palr of electrodes electrlcally isolated from each s ~S~ ~i9~j (85-P-0966) other an electrolyte in contact with the electrodes and means for collectlng electrlcal current therefrom; at least one electrode comprlslng an actlvated carbon electrode having a polyoxometalate compound stabillzed on the activated carbcn surface by a compound capable of generatlng at least one ionlc slte.
The inventlon further relates to an electrlc double layer capacltor comprlslng a houslng at least one palr of spaced-apart actlvated carbon electrodes in the houslng an electrolyte in contact with the electrodes and an lonlcally conductive separator lnterposed between the electrodes and ln contact therewith the electrodes comprlslng activated carbon havlng a polyoxometalate compound and a compound capable of generating at least one ionic slte stabllzed on the activated carbon surface.

Detailed Description of the Inventlon In accordance with the present lnventlon there i5 dlsclosed an lmproved actlvated carbon electrode havlng a polyoxometalate compound adsorbed thereln and stabllized by means of a compound capable of generating at least one ionic site.
Polyoxometalate compounds can be represented by the formula:
Aa~LlMmJ20y]
whereln A is at least one ion selected from the group conslsting of hydrogen the Group I-A to VIII-A or I-B to VII-B elements the rare earths or actlnldes ammonlum alkyl ammonium alkyl phosphonlum and alkyl arsonium;

. 6 1~ 6~6 (85-~-0966) L is at least one element selected from the group consisting of hydrogen and Group III-B to VII-B elements;
M is at least one metal selected from the group consisting of Group I-A to IV-A metals Group VII-A to II-8 metals the rare earths and actinides; and J is at least one metal selected from the group consisting of Group V-A metals and Group VI A metals; and a is a number which when multiplied by the valence of A ~ill balance the charge on the polyoxometalate complex within the brackets 1 is a number ranging from zero to about 20;
m is a number ranging from zero to about 20;
z is a number ranging from about 1 to about 50; and y is a number ranging from about 7 to about 150.
Preferably L is at least one element of the group P As Si Al ~ Ge Ga and B; M is at least one element of the group Zn Ti Mn Fe Co Nl~ Rh Zr and Re; and J is at least one metal of the group Mo ~ and V. Preferably 1 ranges from zero to about 4; m ranges from zero to about 6; 2 ranges from about 6 to about 24; and y ranges from about 18 to about 80.
The species described above comprising J6 octahedra are referred to as isopolyoxometalates. Other elements can be incorporated to llmited extents into the metal oxide lattice structure resulting in species recognized as heteropolyoxometalates. ~any of the Isopolyoxometalate and heteropolyoxometalate complexes are capable of 7 l~S~.6~ ~85-P-0966) sustaining reverslble redox reactlons; being able to transfer or accept from one to more than thlrty-t~o electrons in several well-defined steps over a ~lde voltage range of from about 1.0 volt to about -2.5 volts vs.
a Ag/AgCl reference electrode.
Examples of polyoxometalate compounds include but are not l~mlted to hexametalate anlons ~MmJ6 mY] the Kegg'n an~ons [Ll or 2MmJ12 my' and the Da~son anions ~L2 to 4MmJ18 mY].
A speclfic example of a heteropolyoxometalate is the compound H3PW12040 whlch exhikits a typlcal molecular structure of a Keggin anion which structure is displayed below using a Ball-and-5tick Model and a Coordination Polyhedron Model.

5all ~nd SticK Moc~l 8 ~;~5~ 3~ 5-P-0960 .' ~

Coordin~tlon ~olyhedron Model g ~ Z 5 1 fir:3~ (85-P-0966) Other examples of heteropolyoxometalates having the same structure include H4SiW12040 H3P Mol2040. 5 MolOV20~0 and H4P MollV040 It is understood that these examples are merely illustrative of heteropolyoxometalates and not intended to be limitative of the class of heteropolyoxometalates.
Activated carbon electrodes utilized in energy storage devices generalJy have BET surface areas of from about 100 m2/g to about 2000 m2/g and preferably have ~ET surface areas of from about 500 m2/g to about 1500 m /9. The surface area of act~vated carbon ~s mostly internal and can be produced in most naturally occurring carbonaceous materials by known activation methods. It has been found that the abillty of an activated carbon electrode to store energy is generally proportional to its surface area although the carbon source method of activation and additional processing treatments can also significantly affect the properties of activated carbons.
Polyoxometalate compounds disposed on activated carbon may not be strongly adsorbed that is a portion of the polyoxometalate may merely be weakly held in the micropores of the carbon material and may be easily soluble in electrolyte materials such as sulfuric acid. It is therefore possible for the polyoxometalates to migrate through an activated carbon electrode and electrolyte and pass across a microporous separator which would ~ncrease the leakage current of an energy storage device incorporating such electrode materials therein.
As taught herein a compound having or capable of forming at least one and preferably multi-ionic sites is incorporated into 5~.~i''3~;
(85-P-0966) polyoxometalate-~odified actlvated carbon electrodes to produce greater locali2ation of the polyoxometalate on the carbon by lower~ng lts solubillty. Less soluble polyoxometalates are less llkely to mlgrate throughout an electrolyte thereby reducing the leakage current of a device ln whlch such materials occur.
The replacement of one or more protons of a polyoxometalate complex with a large cation generally reduces the complex s solubllity.
As an example the Cs+ and (C3H7)4N salts of H4PMollV040 are much less soluble in acid solutions such as a sulfuric acid electrolyte solutlon than the fully protonated specles.
In accordance with the present invention a polyoxometalate compound may be disposed more stably onto an activated carbon material through the formation of a material with very low solubility~ such as by using large multisite cations capable of chelating the polyoxometalate in an ionic fashion. The adsorbed polyoxometalate species can be stabllized by 1) ion exchange between a polyoxometalate species already adsorbed onto the carbon and a compound capable of forming at least one ionic site also referred to herein as an ion 2) ion exchange of a soluble polyoxometalate ion from solutlon into a carbon previously doped with the desired ion; 3) by an acid/base reaction between an activated carbon having been made basic or acldic by an adsorbed compound capable of forming at least one ionic site and a polyoxometalate ~hich is acidic or baslc respect~vely; or 4) by an acid/base reaction between an activated carbon having been impregnated with a polyoxometalate compound to become acldlc or baslc and a compound capable of forming at least one lonlc slte whlch ~5 baslc or acldic respectively.

S~
11 (85-P-0966) ~ l~en the polyoxometalate compound is stablized by ion exchange w~th the polyoxometalate species already adsorbed onto activated carbon the exchange may be with any ion that reduces the solubllity product of the polyoxometalate species prevlously adsorbed onto the carbon. This may be accomplished by uslng the same or a different solution for lmpregnation of the carbon with the polyoxometalate species and the chosen ion. Preferably the polyoxometalate ls an anion and the solution electrolyte is acidic. The ion may be formed ln-situ from a neutral base which will protonate in the electrolyte.
The ion is chosen to produce the desired reduced solublllty product and is also an lon which wlll not undergo a faradaic process leading to degradation of the performance of a device in which such electrode material is used. It is further preferred that the ion be multldentate and capable of belng associated with more than one site of an adsorbed polyoxometalate species. Preferably the compound from whlch the lon is obtained is an amine such as pyridine polyvinylpyridlne dlethylenetriamlne dipropylenetriamine and tetraethylenepentamlne.
Other amine compounds suitable for use in such an ion exchange would be obvlous to those s~illed ln the art.
~ hen the polyoxometalate is stabilized by ion exchange of a soluble polyoxometalate lon from solution into a carbon that has been previously doped with the deslred ion the solubility product of the resulting adsorbed polyoxometalate specles is lower than it would have been ln the absence of lon exchange. The actlvated carbon may be doped and lon exchanged with the polyoxometalate in the same or a different electrolyte solutlon.

i25~
12 (85-P-0966~

The polyoxometalate compound may also be stably dlsposed w~th~n the actlvated carbon mlcropores by an acid/base reactlon. In one embodlment, the activated carbon is made baslc by the physlcal adsorptlon of a base, or chemical modification of the carbon surface with a base. A
polyoxometalate acld is then lntroduced to the basic activated carbon to react with the base, ln the Bronsted-Lowry sense, so as to form an adsorbed lon palr comprlslng a polyoxoanlon and the protonated base.
There may also be direct coordination by a donor atom to a peripheral heteroatom in a polyoxometalate compound that possesses an open slte or a weakly bound exchangeable llgand, after the Lewis concept of acidity/baslclty.
It is preferred that the base be multidentate and capable of belng assoclated wlth more than one slte of an adsorbed polyoxometalate specles. In the acid-base reactlon, lt is most preferred that the compound chosen to provlde the base be an amine compound. Examples of amine compounds that are preferred for use in the acid/base reactlon lnclude pyrldlne, polyvlnylpyrldlne, dlethylenetrlamine, dlpropylenetriamine and tetraethylenepentamine. Other amine compounds suitable for use in.such an acid/base reaction ~ould be obvious to those skilled ln the art.
~ he ion-stabllized~ polyoxometalate-modified activated carbon can be used as an electrode in an energy storage device to exhibit greater charge storage capacity than a similar device incorporating an unmodified activated carbon electrode and lower ieakage current than a similar devlce havlng a polyoxometalate-modified actlvated carbon, but ~L25~t~''r?~ (8s p 0966) not lon stabillzed electrode. In the dlscusslon that follows reference ~111 be made to the utllization of lon-stabllzed polyoxometalate-modlfied activated carbon electrodes in an electrlc double layer capacitor as described ln U.5. 3 536 963 to Boos but it ls understood that the advantages due to the lmproved electrode of the present lnventlon are simllarly reallzed in other energy storage devlces that may use actlvated carbon electrodes.

The invention will be more clearly understood with reference to the followlng flgure whereln:
Flgure 1 is an exploded view of the components of a single cell electrlc double layer capacltor.

Detalled Descr!ptlon of the Drawing Referrlng now to the drawing FIG. 1 depicts a double layer capacitor comprlslng a palr of ldentical electrode assemblies 10 11.
Each electrode subassembly includes an electrically conductlng and lonically insulating member 12 whlch can be made of carbon lead iron nickel tantalum conductive butyl rubber or any impervious conducting material. ~ember 12 is characterized by its electrlcal conductlng property and lts chemlcal lnertness to the particular electrolyte employed at the potential lmpressed upon lt. Its prlmary functlons are as a current collector and an lnter-cell lonlc insulator. If the partlcular electronic and ionlc lnsulatlng member ls susceptlble to `"`` 14 ~2S~6~, (85-P-0966) corrosion by the electrolyte or is not completely impervious thus permitting the electrolyte to seep through and corrode ad~olning components the surfaces of the member can be provided with a coating of a noble metal or a substance such as colloidal graphite in a solvent such as alcohol to minimize such problems. This procedure is also effective in reducing leakage currents by better than a factor of 10.
Annular means or gasket 14 is preferably affixed to conducting member 12. Since electrode 13 is not a rigid mass but is to some extent flexible the principal function of gasket 14 ls to confine electrode 13 and prevent the mass of the electrode material from creeping out. The gasket material is preferably an insulator such as butyl rubber although it need not necessarily be that. It should be flexible to accom~odate expansion and contraction of the electrode. Other obvious ways of confining the electrode ~ould be apparent to those skilled in the art.
Separator 15 is generally made of a hlghly porous material whlch functions as an electronic insulator between the electrodes yet afford~ng free and unobstructed movement to the ions in the electrolyte. The pores of the separator 15 must be small enough to prevent carbon-to-carbon contact between the opposing electrodes since such a condition would result in a short circuit and consequent rapid depletion of the charges accumulated on the electrodes. The separator can also be a nonporous ion-conducting material such as an ion exchange membrane. Any conventional battery separator should be suitable and materials such as porous polyvinyl chloride glass fiber filter paper porous polypropylene cellulose ace.tate and mixed esters of cellulose may be ~5~6~6 (85-P-0966) used. Prior to its use the separator is generally saturated with electrolyte~ Thls can be accomplished by soaking the separator in the electrolyte for about 15 mlnutes or less.
Carbon electrode 13 in accordance w~th the present invention comprises activated carbon having a polyoxometalate capable of multiple reverse redox reactions adsorbed therein and stabilized with an amlne compound, as well as an electrolyte. The activated carbon may be imbued with the electrolyte elther before or after it is modified ~ith the polyoxometalate compound. Likewise, the activated carbon may also be simultaneously exposed to both the electrolyte and the polyoxometalate compound, wlthout suffering adverse charge storage characteristics to any of the components. The preferred method may vary with various polyoxometalates and the polyoxometalate is stabilized with a compound capable of formlng at least one ionic site in accordance with the process taught herein.
The electrolyte should consist of a hlghly conductive medium such as an aqueous solution of an acid, salt or base. Examples of suitable aqueous electrolytes are: ammonium chloride, sodium chloride, calcium chloride, potassium chloride, potassium carbonate, sulfuric acid, fluoroboric acid, sodium hydroxide, potassium hydroxide, etc. The pH of the solution must be chosen so that the polyoxometalate remains stable as used. The pH may vary with various polyoxometalates.
The electrolyte in the electrode structure serves three functions: (1) as a promoter of ion conductivity, (2) as a source of lons, and (3) as a binder for the carbon particles. Sufficient 16 (85-P-0966) electrolyte should be used to accomodate these functions. A separate binder can be used to perform the electrolyte s binder function however the separate binder would add an element of resistance which is undesirable.
The pressure applied to form the electrodes is dependent on many variables such as the dimensions of the electrode particle size of the carbon material and particular electrolyte used. It should be limited to leave an amount of electrolyte within the electrode structure sufficient to accompllsh its three functions referred to above.
A pair of electrodes thus produced are placed within a separate annular member 14 which is affixed to a circular plate 12. A separator membrane ls interposed between the two electrodes and this capacitor cell is disposed on a lower platen of a press. The upper platen is brought down upon the cell until the surfaces make contact and a concentric ring is sllpped over the cell. At this point the capacitor cell ls confined by the upper platen the lower platen and the concentric ring. The cell is then compressed at a pressure sufficient to render the cell a coherent structure. Pressure on the order of about 2 0~0 p.s.i. has been found sufficient.

Examples The following examples demonstrate the increased charge storage ability and low leakage currents of activated carbon electrodes that have been modified with polyoxometalates which were stabilized on the activated carbon with compounds capable of forming at least one ionic site.

17 ~S~6~ (8s-p-o966) Example 1 This example illustrates the sub~ect lnventlon whereby the polyoxometalate is stabilized by an amine through an ion exchange process. Four dlfferent amines are used in separate tests to show their abll5ty to stabil5ze the polyoxometalate. Two controls one in which the activated carbon is unmodified and one in which the activated carbon is mod~fled with a polyoxoanion but not stabilized with an amine are also presented for comparison. In each test the carbon was utilized as the electrode material ln an electric double layer capacitor.
The carbon used in the electrodes of this Example was an activated carbon commercially available under the tradename PWA from the Calgon Carbon Corporation. Thls carbon had a 8ET surface area of about 1142 m2/g and a pore dlameter (Angstrom) to pore area (m2/g) distribution of about as follows: <20/1059; 20-30/51; 30-40/11; 40~50/3;
and 60-300/12.
The polyoxometalate used to modlfy thls carbon was H4PMollV040. This polyoxometalate was stabilized with various amlnes in the following ion exchange manner:
A carbon slurry was prepared by mixing from about 149 to about 209 of PWA carbon with 25 weight percent H2S04 until a clear H2S04 solution layer was observed on settling of the carbon. The carbon was soaked in thls manner for about five days. The carbon slurry was then st~rred v59Orously while H4PMollV040 prepared by the method of Tslgidnos and Hallada (Inorgan5c Chemistry Vol. 7 pp.
437-441 1968) was added. A blue color developed immediately signlfying ` 18 ~25~6~t; ~85-P-0966) that some reductlon of H4PMollV040 was occurrlng. Thls slurry was allowed to soak for about five addltlonal days ~lth occaslonal stirring then filtered to remove the blue liquid phase. A slurry was then reformed with the polyoxometalate-modlfled carbon suspended ln 25 weight percent H2S04 To such a solution was then added one of the followlng amines; 1.
ml of ninety-five weight percent diethylenetriamine (DETA) 1 ml. of reagent grade pyridine 0.28 9 polyvlnylpyrld~ne (PVP) dissolved in 25 ml of H2S04 or 1 ml of technlcal grade tetraethylenepentamine (TEPA).
The carbon was allowed to soak under the above conditions for about one additional day with occassional stirring. The carbon was then isolated by filtering or decanting off the solution phase handpressed between sheets of adsorbant paper and then further pressed between sheets of adsorbent paper in a dle at about 6 000 psi RAM force. The carbon was then grated through a 20 mesh screen. This modified carbon material was then processed lnto an electrode and assembled lnto a capacitor cell slmllar to that shown ln Figure 1. Unmodified P~A carbon in as-received condition was processed into an electrode in accordance with the above technique with the exception that no polyoxometalate solution or amine was lncorporated into the carbon slurry and assembled into a capacitor cell similar to the capac~tor shown in Figure 1. P~A
carbon was also processed lnto an electrode as described above but omlttlng the step of adding an amine compound to the solution and used to fabrlcate a capacitor cell slmllar to that shown ln figure 1.

``" 1~S1~6 '9 ~85-P-0966) The fabrication of the electrodes and capacitors were as follows; electrode pellets each about 2.86 cm (1.125 inches) diameter by about 0.32 cm (0.125 inch) thick and containing about 5.2 9 of the selected carbon were pressed from a die at 6 000 psi RAM force. The pellets. were loaded into gaskets of butyl rubber. The gaskets were sealed on one face with a disk of conductive butyl rubber to serve as a current collector. This was affixed to the gasket with an adhesive.
Each pellet was then uniformly wetted with about 0.2 ml of 25 weight percent H2504. A dry porous polypropylene membrane was interposed between a pair of slmilar electrode assemblies to form a cell. The membrane was sealed to each butyl rubber gasket with adhesive. Each completed cell similar to the cell shown in Figure 1 was placed between brass contacts in a compression flxture at 3 000 psi RAM force.
Electrical connections were provided between the cell and a power supply without regard to polarity since both half-cells were equivalent.
However polarity was always mainta~ned the same ~n all tests w~th each cell.
Parameters characterizing the performance of six such cells; one utilizing control unmodified activated carbon electrodes one incorporating polyoxometalate-modified activated carbon electrodes and four using polyoxomltalate-modified and amine-stabilized activated carbon electrodes at a charging potential of 1.0 volt and an ambient temperature of approximately 20-25C are listed in Table 1.

(85-P-0966) Electrical Properties of Capacitors Having Varlous Modified Capacitor Electrodes CHARGING DAYS LEAKAGE
CAPACITOR TIME ON CURREHT CAPACITANCE
ELECTRODE CYCLE (hrs.)TEST (ma)(farads/~) UnmQdlfled 1 17 1 0.42 19.35 Carbon Control 2 92 5 0.19 --Polyoxometalate 1 17 1 11.0 33.4 Modified Carbon 2 92 S 7.0 --Polyoxometalate 1 20.5 1 5.7 32.6 And DETA Modlfled 2 88 5 0.34 31.9 Carbon 6 92 12 0.61 --Polyoxometalate 1 88 5 1.3 33.9 and Pyridlne Modified Carbon Polyoxometalate 1 20.5 1 4.3 29.7 And PVP Modlfled 2 17 5 1.7 28.5 Carbon 6 92 12 2.6 --12 120 24 2.3 27.5 Polyoxometalate 1 88 5 0.10 34.5 and TEPA Modifled 3 92 12 0.10 30.9 Carbon 183 48 23 0.30 29.8 As can be seen from Table 1, the capacitor having polyoxometalate-modified carbon electrodes possesses a much greater charge storage capacitance (33.4 farads/g) than the capacitor using unmodifled carbon (19.35 farads/g), but at the expense of greatly lncreased leakage current (7-11 ma versus 0.19-0.42 ma, respectlvely).
The additlon of a compound having at least one lonic slte, such as pyrldlne, or multlple lonic sltes, such as DETA, PVP and TEPA, to 21 i2S~ (85-P-0966) stablllze the polyoxometalate-modlfied carbon does rot significantly affect the charge storage capacity of such a materlal but as shown ln Table 1 does reduce the leakage current of an energy storage devlce that incorporates electrodes uslng these materials.
Example 2 This example demonstrates the sub~ect invention whereby the polyoxometalate compound ls stabllized by an amine through an acld/base reaction. Three tests were conducted for this Example; one control test wherein an electric double layer capacltor was fabricated uslng two unmodified carbon control electrodes a second control test ~herein a capacitor was fabricated using carbon electrodes that were modified by the polyoxoanlon H3PMol2040 and one test wherein the carbon electrodes for the capacltor were modlfled by the polyoxoanion used in the above test run and further modlfied by an acid/base reaction with the amine dlpropylenetrlamine (DPTA).
The carbon used in each of the electrodes of this Example was an actlvated carbon commercially available under the tradename Wltco 950 from the Witco Chemical Company. Thls carbon had a BET surface area of about 1076 m2/g and a pore dlameter (Angstroms) to pore area (m2/g) distrlbution about as follows: <20/1033; 20-30/37; 30-40/2; 40-50/1;
50-60/0.4 and 60-300/3.
The unmodlfied control carbon was processed into an electric double layer capacitor as taught in Example 1. The electrlc double layer capacitor control having H3PMol20~0 polyoxometalate-modified carbon electrodes was prepared in the same manner as taught in Example 1 for polyoxometalate-modlfled carbon electrode capacitors.

22 (85-P-0966) the capacitor havlng the DPTA and polyoxometalate modlfied carbon electrodes was fabrlcated from electrodes having been exposed to an acld/base polyoxometalate/amine reactlon whlch proceeded as follows:
About 0.25g of DPTA was dissolved in about 20 ml of methanol.
This solutlon was then admlxed with about 259 of Wltco 950 carbon to form a thick paste. The paste was drled at about 75C for about 45 minutes.
About 6.259 of the polyoxometalate H3PMol2040 was separately dissolved ln about 299 of methanol which resultant solution was then mixed with the DPTA-lmpregnated carbon. The solvent was allowed to evaporate at about 100C and the carbon was further washed with methanol. Thls process was repeated with the exception that the 0.259 of DPTA dissolYed ln methanol was increased to l.Og of DPTA in 20 ml of methanol. The resultant carbon from this procedure was found to contaln about 35 welght percent polyoxometalate.
The carbon was then used to fabrlcate electrodes and a capacltor as taught ln Example 1 above. Characterlstics of the cap2citors formed ln Example 2 are shown in Table 2. As can be seen the capacltor havlng amine-stabillzed and polyoxometalate-modified carbon electrodes exhibits a capacitance that is comparable to the capacitor havin~ only polyoxometalate-modified carbon electrodes and the for~er also has a significantly lower leakage current than the latter.

~S~ 6 23 (85-P-0966) Electrical Propertles of Capacitors Having Various Modifled Capacltor Electrodes CHARGING LEAKAGE
CAPACITOR TIME CURRENT CAPACITANCE
ELECTROD (hrs.~ (ma~ (farads/q~
Unmodified Carbon Control 69 0.43 22 H3dMfl24C b 92 3.7 43 DPTA Modi~ied Carbon 69 1.1 42 S
24 (85-P-0966) It ls to be understood that the foregolng examples have been provlded to enable those skllled ln the art to have representatlve examples by whlch to evaluate the invention and that these examples should not be construed as any limitatlon on the scope of thls lnvention. Inasmuch as the composition of the modified actlvated carbon electrodes employed in the present lnventlon can be varled wlthin the scope of the total speclfication dlsclosure neither the partlcular lonic slte compound polyoxometalate or activated carbon components nor the relative amounts of the components ln the electrodes exemplIfled hereln nor the exempllfied technlques for disposing the lonic site compound and polyoxometalate onto the carbon shall be construed as limitations of the inventlon.
Thus it is believed that any of the variables d~sclosed herein can readlly be determined and controlled wlthout departing from the spirit of the invention hereln dlsclosed and descrlbed. Moreover the scope of the lnvention shall lnclude all modificatlons and varlations that fall wlthin the scope of the attached clalms.

Claims

1. An improved carbon-based electrode, which electrode comprises an activated carbon electrode having a polyoxometalate compound stabilized on the activated carbon surface by a compound capable of generating at least one ionic site.

2. The electrode in accordance with claim 1 wherein said polyoxometalate compound is represented by the formula:
Aa[L1MmJzOy]
wherein A is at least one ion selected from the group consisting of hydrogen, the Group I-A to VIII-A or I-B to VII-B elements, the rare earths or actinides, ammonium, alkyl ammonium, alkyl phosphonium and alkyl arsonium;
L is at least one element selected from the group consisting of hydrogen and Group III-B to VII-B elements;
M is at least one metal selected from the group consisting of Group I-A to IV-A metals, Group VII-A to II-B metals, the rare earths and actinides; and J is at least one metal selected from the group consisting of Group V-A metals and Group VI-A metals and a is a number which when multiplied by the valence of A will balance the charge on the polyoxometalate complex within the brackets;
1 is a number ranging from zero to about 20;

m is a number ranging from zero to about 20;
z is a number ranging from about 1 to about 50; and y is a number ranging from about 7 to about 150.

3. The electrode in accordance with claim 2 wherein L is at least one element of the group P,As,Si,Al,H,Ge,Ga, and B; M is at least one element of the group Zn,Ti,Mn,fe,Co,Ni,Rh,Zr and Re; and J is at least one metal of the group Mo, W and V; and wherein 1 ranges from zero to about 4;
m ranges from zero to about 6;
z ranges from about 6 to about 24; and y ranges from about 18 to about 80.

4. The electrode in accordance with claim 1 wherein said activated carbon has a BET surface area of from about 100 m2/g to about 2000 m2/g.

5. The electrode in accordance with claim 1 wherein said activated carbon has a BET surface area of from about 500 m2/g to about 1500 m2/g.

6. The electrode in accordance with claim 1 wherein said compound capable of generating at least one ionic site is an amine compound.

7. The electrode in accordance with claim 6 wherein said amine is selected from the group consisting of pyridine polyvinylpyridine diethylenetrlamine dipropylenetriamine and tetraethylenepentamine.

8. An energy storage device comprising a pair of electrodes electrically isolated from each other an electrolyte in contact with the electrodes and means for collecting electrical current therefrom; at least one electrode comprising an activated carbon electrode having a polyoxometalate compound stabilized on the activated carbon surface by a compound capable of generating at least one ionic site.

9. An electric double layer capacitor comprising a housing at least one pair of spaced activated carbon electrodes in the housing an electrolyte in contact with said electrodes, and an ionically conductive separator interposed between said electrodes and in contact therewith the electrodes comprising activated carbon having a polyoxometalate compound stabilized on the activated carbon surface by a compound capable of generating at least one ionic site.

10. A process for making an improved carbon-based electrode comprising contacting a polyoxometalate compound and a compound capable of generating at least one ionic site in the presence of activated carbon so as to stably dispose the polyoxometalate on the activated carbon.

11. The process in accordance with claim 10 wherein said contacting occurs by ion exchange.

12. The process in accordance with claim 11 wherein said polyoxometalate compound is adsorbed on the activated carbon prior to ion exchanging.

13.. The process in accordance with claim 114 wherein said compound capable of generating at least one ionic site is adsorbed on said activated carbon prior to ion exchanging.

14. The process in accordance with claim 11 wherein said compound capable of generating at least one ionic site is multidentate.

15. The process in accordance with claim 11 wherein said compound capable of generating at least one ionic site is an amine compound.

16. The process in accordance with claim 15 wherein said amine is selected from the group consisting of pyridine, polyvinylpyridine, diethylenetriamine, dipropylenetriamine and tetraethylenepentamine.

17. The process in accordance with claim 10 wherein said contacting occurs by an acid/base reaction.

18. The process in accordance with claim 17 wherein said compound capable of generating at least one ionic site is a multidentate compound.

19. The process in accordance with claim 17 wherein said compound capable of generating at least one ionic site is an amine compound.

20. The process in accordance with claim 19 wherein said amine is selected from the group consisting of pyridine polyvinylpyridine diethylenetriamine dipropylenetriamine and tetraethylenepentamine.